A fuel cell is usually formed to have, as one unit, a cell wherein: a membrane electrode assembly, which may be abbreviated to an MEA hereinafter, is composed of electrodes of an anode and a cathode, in which reaction for generating electricity is caused, and a polymer electrolyte membrane, which becomes an ion conductor, between the anode and the cathode; and the MEA is sandwiched between separators. The electrodes are composed of: electrode substrates for promoting gas diffusion and performing power collection (feeding), which may be referred to as gas diffusion electrodes or current collectors; and electrocatalyst layers of the anode and the cathode, which are actual electrochemical reaction fields. For example, in the anode of a polymer electrolyte membrane fuel cell, which may be abbreviated to a PEFC hereinafter, a fuel such as hydrogen gas reacts in its anode catalyst layer so as to generate protons and electrons, and the electrons are conducted to its electrode substrate and the protons are conducted to its polymer electrolyte. For this reason, the anode is required to be good in gas diffusivity, electron conductivity and ion conductivity. On the other hand, in the cathode thereof, on its cathode catalyst layer, an oxidizing gas such as oxygen or air reacts with the protons conducted from the polymer electrolyte and the electrons conducted from the electrode substrate so as to generate water. For this reason, the cathode is required to have gas diffusivity, electron conductivity and ion conductivity, and further it becomes necessary to exhaust the generated water therefrom effectively.
Of polymer electrolyte membrane fuel cells, a direct methanol fuel cell, which may be abbreviated to a DMFC hereinafter, wherein an organic solvent such as methanol is used as a fuel, is required to have performances different from those of any conventional. PEFC, wherein hydrogen gas is used as a fuel. In other words, in the DMFC, a fuel such as an aqueous solution of methanol reacts on its anode catalyst layer in the anode, so as to generate protons, electrons and carbon dioxide. The electrons are conducted to its electrode substrate, and the protons are conducted to its polymer electrolyte. The carbon dioxide passes through the electrode substrate to be exhausted to the outside of the system. Therefore, the DMFC is required to have the permeability of a fuel such as an aqueous solution of methanol and the exhaustability of carbon dioxide as well as properties required for the anode electrode of any conventional PEFC.
In conventional MEA's, a product wherein fine metal particles having catalytic power are carried on carbon to make the surface area of the metal catalyst large is used in many cases (see the following Non-patent document 1 and Non-patent document 2). When carbon is used as a catalyst-carrying body as described above, the viscosity of a coating solution of the catalyst is easily adjusted; thus, a layer made of the catalyst is easily formed. As the amount of the fine metal particles carried on carbon is larger, the reaction efficiency per unit area is better. If the amount of the fine metal particles is made too large, the diameter of the fine metal particles becomes large so that the surface area becomes small, thereby lowering the catalyst efficiency. For this reason, there is a limit to the amount of the particles that can be carried. When such a catalyst carried on carbon is used, the catalyst layer becomes thick since the volume of carbon is large. In the DMFC, oxidizing reaction of methanol is not easily caused. Thus, a large amount of a catalyst is required so that a layer made of the catalyst becomes thicker.
(Non-Patent Document 1)
Nakagawa et al., “Production of Liquid Supplying DMFC and Performance Analysis thereof”, The Electrochemical Society of Japan, Summaries of the 69th Lectures, p. 69
(Non-Patent Document 2)
Fukunaga et al., “Anode Electrode Structure of Gas Supplying DMFC, and Overvoltage therein”, The Electrochemical Society of Japan, Summaries of the 69th Lectures, p. 76